Device-to-Device (D2D) or sidelink communications and cellular- or network-controlled D2D (sidelink) communications are expected to play an important role in the next generation wireless networks D2D technology can be used to provide diversity in space, time and frequency, and increase the robustness against fading and interference. User Equipment (UE) cooperation based on D2D is a technology that is receiving attention. With advances in D2D communications, UE cooperation is expected to play an important role in the future of wireless communication systems such as Long Term Evolution (LTE), also known as the fourth Generation (4G); enhanced LTE, (eLTE) advanced or fifth Generation (5G) and so on. Hereinafter by D2D communications is also meant sidelink communications.
One object with group transmission using D2D (“cooperative D2D”) may be a way to increase the coverage and user bit rate for example in the future high frequency 5G network. As mentioned, UE cooperation based on D2D, or, in other words, cooperative D2D, is gaining traction in the industry. A concept how group transmission using D2D can be performed was developed recently. According to the concept, the respective UEs in the group of UEs transmit synchronized as one antenna array in order to increase the UpLink (UL) coverage and bit rate. The UEs first transmit the data within the group using D2D methods and then transmit the same data jointly to the network node.
With group transmission, the basic idea is that the network node or base station sees the group as one single UE. All UEs have the data in their group transmission Hybrid Automatic Repeat ReQuest (HARQ) buffers and their transmissions completely synchronized. Since the UL transmissions rely on HARQ feedback, in the form of an ACKnowledgment (ACK) or Non-ACK (NACK), reception of the feedback need to be done by all UEs and also interpreted the same way by all UEs.
The HARQ protocol is widely used in 3G and 4G systems and will undoubtedly be used also in 5G systems to provide fast re-transmissions on the Medium Access Control (MAC) layer. As explained above, HARQ is used both in the UL and in the DL and may be configured in different ways, e.g. the maximum number of re-transmissions, operating BLock Error Rate (BLER), when and how to retransmit etc. Compared to the unicast transmission scheme, group transmission (multicast within a group) is more efficient in terms of the resource consumption. There are a few ways to do group transmission. One option is to do group transmission using D2D as explained earlier, the D2D interface is referred to PC5 interface in LTE. Another other option is to use Multimedia Broadcast Muti Multicast Service (MBMS) techniques. It is a network (NW) entity or a network node, e.g., a base station that does the group transmission using the cellular air interface, i.e., Uu. In order to do group transmission, support of the user feedback and HARQ with retransmission functionality is important for the transmitter to achieve reliable and efficient group transmission. It is under discussion in 3GPP to introduce both features in group transmission techniques, like Vehicle to Vehicle (V2X e.g. Vehicle to Vehicle (V2V)) Work Item (WI) in 3GPP (see references Ericsson R1-162831 entitled “Uu Enhancements for V2X” and Ericsson R2-162815 entitled “Other MBMS Enhancements for V2X”).
As described above, a special situation where the HARQ protocol could be applied is a Single Cell Point To Multipoint (SC-PTM) transmission in V2X communication. This is assumed to be a complementary bearer type for MBMS transmission (in addition to MBSFN transmission), i.e. the Release-12 MBMS architecture is used. The SC-PTM transmission may be seen as transferring a MBMS session using the Physical Downlink Shared Channel (PDSCH). For each MBMS session, it is normally only one scheduling entity per cell due to there is usually one group of users (UEs) defined for one MBMS session/service. The group receives the MBMS data from BM-SC, then forwards/distributes data in the cell via PDSCH in the allocated sub-frames, which may be predefined. This scheduling entity behaves like a normal single UE scheduling entity concerning the scheduling and transmission procedure. The SC-PTM, or MBMS (MBSFN) are the candidate multicast/broadcast transmission techniques for V2X when LTE cellular link based delivery option is chosen.
A way to implement the HARQ protocol is to use autonomous re-transmissions, i.e. the transmitter always performs a given number of HARQ retransmission attempts. Autonomous retransmissions are especially suitable in one-to-many communication scenarios since using HARQ feedback from many recipients is complicated. With suitable setting for the number of HARQ transmission attempts, most of transmission errors can be recovered. FIG. 1 depicts autonomous re-transmissions.
IN FIG. 1, n is the Transmission Time Interval (TTI) number. The Figure also visualizes a NetWork node (NW); UE1; UE2 and UEN. The NW node is configured to execute a given number of re-transmissions regardless whether or not the group of UEs succeeds in receiving the re-transmissions. A re-transmission (by the NW node) occurs immediately after the first transmission as shown; UEs close to the NW node are able to successfully decode the receive message included in the re-transmission. In FIG. 1, UE1 has succeeded the decoding (OK for each transmission and re-transmission). Bu UEs at cell-edge (here UE2 an d UEN) may not always be able to decode the message even with a re-transmission message. This is shown with FALSE for UE2 and UEN.
The FIGS. 2-4 depict a V2X simulation using single cell point to multipoint (SCPTM) to transmit a Cooperative Awareness Message (CAM). CAM is one of the components of the reference architecture defined by the European Telecommunication Standards Institute (ETSI) for transmitting geographically aware information with relevant date for other vehicles. The simulation is for an LTE network with 10 MHz bandwidth. FIG. 2 shows the transmission reliability of the CAM without using HARQ operations to support link adaptation and retransmissions according to the radio channel quality.
For example, in FIG. 2, there are a few percent of the users with a SINR below 0 dB, which cannot ensure transmission reliability with only 1 transmission attempt.
Since it is challenging to guarantee all vehicles in the area (cells) with SCPTM like transmission schemes transmission reliability, it is natural to introduce the support of HARQ functionalities which support retransmissions. However, using retransmissions to achieve reliable transmission introduces additional delays.
A problem with autonomous re-transmissions is that some resources are wasted due to unnecessary re-transmissions where the group transmission has already succeeded in the first transmission attempt. Also, it is generally difficult to find a suitable setting due to the UEs in the group might have very different radio link status and different mobility status. The conditions may also change dynamically. FIG. 3 shows how the successful reception rate increases with the number of configured retransmissions in a V2X scenario. The drawback of configuring a high number of autonomous retransmissions is shown in FIG. 4 which depicts the end-to-end delay for different number of configured re-transmissions. It is clear that there is a trade-off between successful reception ratio and end-to-end delay.
In 5G-based NX Radio Access Technology (RAT) employing higher frequencies, the challenging propagation conditions at higher frequencies essentially Line-Of-Site (LOS) to the network node e.g. an eNode B or a base station is needed for good radio conditions. This suggests that which UEs in a group that will have good radio conditions will change more rapidly over time compared to that in LTE. It also implies that autonomous retransmissions in the NX RAT will require a high number of re-transmissions.
Utilizing D2D within the group can be exploited to increase the performance of autonomous retransmissions as previously described. In the NX RAT scenario this could be especially fruitful since there will be users with LOS between one another and where one UE have LOS to NW node and another UE which does not have LOS to the NW node. Hence, one UE will have a good channel to the NW node and to a UE which has a bad channel to the NW node. FIG. 5 illustrates the situation where UEs have LOS to each other but not all have LOS to the NW node.
Furthermore, the scenario with autonomous retransmissions is a situation, perhaps the only, where a majority of the UEs know exactly what will be retransmitted.
The current group transmission technique is not sufficient to deliver V2X messages efficiently and it is meaningful to investigate how to improve the autonomous group HARQ transmission schemes by combining the eNode Bs (NW nodes) retransmissions with D2D (sidelink) transmissions between the UEs.